Q 1 :

Three vectors P→,Q→ and R→ are shown in the figure. Let S be any point on the vector R→. The distance between the points P and S is b|R→|. The general relation among vectors P→,Q→ and S→ is            [2017]

  • S→=(1-b)P→+bQ→

     

  • S→=(b-1)P→+bQ→

     

  • S→=(1-b2)P→+bQ→

     

  • S→=(1-b)P→+b2Q→

     

(1)

Here P→+bR→=S→ ⇒ R→=S→-P→b

Also R→=Q→-P→

∴  S→-P→b=Q→-P→ ⇒ S→-P→=bQ→-bP→

∴  S→=bQ→+(1-b)P→



Q 2 :

Two vectors A→ and B→ are defined as A→=ai^ and B→=a(cosωt i^+sinωt j^), where a is a constant and ω=π6 rad s-1. If |A→+B→|=3|A→-B→| at time t=τ for the first time, the value of τ, in seconds, is ______.                 [2018]



(2)

Given: |A→+B→|=3|A→-B→|

∴|ai^+acosωt i^+asinωt j^|=3|ai^-acosωt i^-asinωt j^|

⇒|(1+cosωt)i^+sinωt j^|=3|(1-cosωt)i^-sinωt j^|

          2+2cosωt=32-2cosωt

∴ 1+cosωt=3(1-cosωt)

⇒4cosωt=2  ∴  cosωt=12         or, ωt=π3

∴ π6×τ=π3  ∴  τ=2.00 seconds